Diagnostic imaging systems such as computed tomography (CT) demand high power and high resolution. Higher power X-ray tubes allows an imager to image denser materials with less exposure time and this can be extremely beneficial for injured patients who must remain stationary during the imaging process. Higher resolution imagers allows for greater detail in the object being imaged which can aid in patient diagnosis. Therefore, an X-ray tube that provides for both higher power and higher resolution is more desirable over lower powered lower resolution replacements.
Unfortunately, higher X-ray tube power increases the temperature of the X-ray tube's anode and this higher temperature can lead to X-ray tube failure unless mitigation techniques are utilized to reduce the heat before damage occurs. One method to reduce anode heating is by rotating the anode in the X-ray tube to spread the heat caused by the electron beam impacting the surface of the anode across the surface of the anode. By reducing the localized heating on the anode, higher X-ray tube powers can be achieved.
A common method to increase the resolution of imaging systems using digital detectors is by oversampling. To achieve oversampling, the focal spot is moved between two successive views on the anode using electrostatic or magnetostatic means. If the electron beam is deflected with or against the direction of the target anode, at the focal spot location, the deflection is referred to as x-wobble or x-deflection. Focal spot motion in the +x direction coincides with the direction of the target surface motion while focal spot motion in the −x direction is opposite to the direction of the target surface motion.
FIG. 1 is a perspective view of the components inside a typical X-ray tube that utilizes focal spot deflection. Typically, a high voltage power supply 102 supplies filament voltage 104 to filament 106 in an X-ray tube causing the filament 106 to heat up and boil off a stream of electrons 108. The electron beam 108 is drawn across the X-ray tube by the positively charged anode 110. The electron beam 108 impacts a small area on the target surface of the anode 110 called the focal spot. The interaction with the target material results in an X-ray beam.
Steering an electron beam using electrostatic mean is typically accomplished by arranging several electrodes 112, 116, 126, 128 in close proximity to the electron beam 108. Typically, the electrodes 112, 116 are energized to shape and deflect the electron beam 108 as the beam leaves the cathode 106 to two or more distinct locations 120, 122 on the anode depending on the bias applied to a particular electrode. In reference to FIG. 1, applying specific bias potentials to a first electrode 112 and a second electrode 116 will cause the electron beam 108 to move to distinct positions on anode 110. The magnitude of the beam movement is directly related to the magnitude of the bias applied to the electrodes. If the first bias voltage 114 on the first electrode 112 is greater than the second bias voltage 118 on the second electrode 116, electron beam 108 will move to the left or −x direction to a first focal spot position 120. Alternatively, if the bias voltage on the second electrode 116 is greater than the bias voltage on the first electrode 112, the beam will move to the right or the +x direction or to a second focal spot position 122. The magnitude of the electrode bias voltages and the position of the electrode with respect to the electron beam will determine the focal spot location.
Additionally, magnetostatic means can be used to steer the electron beam by placing magnets near the path of the electron beam. Varying the strength, polarity and position of the magnets with respect to the electron beam will determine the location of the focal spot on the anode if magnetostatic focal spot control is used.
FIG. 2 is a graph illustrating a heating and cooling cycle for a particular point in the focal spot on the anode when there is no focal spot deflection. When a particular location in a rotating anode tube enters the electron beam impacts region, the impact temperature at this location begins to rapidly increase. After this target location rotates out of the impact region with the electron beam or the electron beam is turned off, the localized temperature decreases as the location begins to cool.
When the anode is rotated to reduce anode heating and focal spot deflection to increase resolution are combined, creating an additional heating cycle is possible. If the focal spot is deflected in the same direction as the rotation of the anode 110, the +x direction, by simultaneously switching the bias voltage 114, 118, on the electrodes 112, 116, it is possible to cause increased anode heating shown in FIG. 3 if the transition time, anode rotation frequency, deflection distance and target radius are selected such that the relative speed between the target surface and the electron beam impact area is sufficiently small. The region on the target that is impacted by the electron beam is characterized by the area between the solid lines in FIG. 3. During the transition time tx the slope of the solid line equals the slope of the stitched lines. This represents the situation where the relative speed between the target surface and the electron beam impact area is zero. This represents a typical situation where the transition time is in the order of a few microseconds. Different transition times will influence the final anode temperature reached. However transition times much shorter than one microsecond are impractical due to design limitations of the voltage switching circuits, and much larger transition times are undesirable from an application standpoint due to loss of image information per unit time.
Point 302 on the anode 108 has a trajectory that remains within the impact area on the nominal focal spot radius 124 between the focal spot's static time tS1 at a first position 120, through the transition tx and during the static time tS2 at the second position 122. Without deflection the total time for any point on the target to remain under the electron beam would be tS1+tS2. The anode point 302 heats up as the impact area is bombarded at the first position 120 during tS1, point 302 is then further heated during the transition period tx and finally point 302 is heated during heating cycle 304 at the second position 122 during the static time tS2.
The additional heating cycle during the transition period tx for point 302 limits the maximum power the electron beam is allowed to carry and forces the user to decrease the X-ray tube's power such that the impact temperature remains below the X-ray tube manufacturer's maximum rated impact temperature of the X-ray tube. If the X-ray tube's power is not de-rated to prevent exceeding the maximum allowable operating temperature, the anode temperature may exceed the recommended maximum limits of the manufacture and damage to the anode can occur, leading to failure of the X-ray tube.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a method for reducing X-ray tube power de-rating during dynamic focal spot deflection caused by anode heating.